With the colder weather upon us, the central heating has been switched on
again and it's going to be another month or two before the radiators in
question are reconnected. How long is it acceptable to run the system
without inhibitor in it. If necessary I could put it in this weekend, but
if
it's not reeally necessary for a couple of months then I'll wait.

It depends. Some people run without inhibitor, although can be a disaster if
serious corrosion sets in. I'd probably not want to run several months
without it, although I would be happy for a week if there was no chance of
freezing. You could buy some "value" inhibitor in the meantime, particularly
if a "couple of months" turns into "a few years" due to the chronic tuit
shortage.

Toolstation will do you a dose of inhibitor for 3.35 inc VAT.

http://www.toolstation.com/index.html?code=94504

Christian.